1. Field of Invention
The present invention relates to a system for purifying an exhaust gas and a method for controlling the same. More particularly, the present invention relates to a system for purifying an exhaust gas and a method for controlling the same which releases nitrogen oxides from a lean NOx trap (LNT) catalyst and reduces or slips the released nitrogen oxides so as to use the released nitrogen oxides for regeneration of a particulate filter by creating a rich atmosphere according to a temperature of the LNT catalyst and a temperature of the particulate filter.
2. Description of Related Art
Generally, exhaust gas flowing out through an exhaust manifold from an engine is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe.
A denitrification catalyst (DeNOx catalyst) is one type of such a catalytic converter and purifies nitrogen oxides (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst is used as such a DeNOx catalyst. The LNT catalyst absorbs the NOx contained in the exhaust gas when the engine operates in a lean atmosphere, and releases the absorbed NOx when the engine operates in a rich atmosphere.
Meanwhile, a particulate filter for trapping particulate matters (PM) contained in the exhaust gas is mounted on the exhaust pipe. If excess soot is trapped in the particulate filter, it is difficult for the exhaust gas to pass the particulate filter and a pressure of the exhaust gas increases. If the pressure of the exhaust gas is high, engine performance may be deteriorated and the particulate filter may be damaged. Therefore, if an amount of the soot trapped in the particulate filter is larger than or equal to a predetermined amount, a temperature of the exhaust gas is raised so as to burn the soot trapped in the particulate filter. Such a process is called a regeneration of the particulate filter.
The regeneration of the particulate filter is divided into a passive regeneration and an active regeneration. The passive regeneration is performed by supplying the nitrogen dioxides (NO2) required for regenerating the soot from the nitrogen oxides contained in exhaust gas. On the contrary, the active regeneration is performed by post-injecting fuel to a combustion chamber of the engine. In the active regeneration, the post-injected fuel is oxidized so as to generate oxidation heat and the soot trapped in the particulate filter is burnt by the oxidation heat. In a case of the active regeneration, since a regeneration temperature is high, large energy is required for entering the regeneration process. Therefore, regeneration period is long so as to prevent energy consumption. In the passive regeneration, compared with the active regeneration, a regeneration temperature is low and thus small energy is required for entering the regeneration process. Therefore, regeneration period is short.
If the active regeneration is performed too frequently, fuel consumption is deteriorated and oil dilution occurs. To solve such problems, a volume of the particulate filter needs to be larger than 2.5 L.
In a case that the LNT catalyst, however, is disposed at an upstream of the particulate filter, the nitrogen dioxides (NO2) contained in the exhaust gas are absorbed at the LNT catalyst and thus the passive regeneration of the particulate filter is hindered.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.